1. Field of the Invention
The present invention relates to a process for producing a UV ray cross-linked 1,2-polybutadiene foamed products having a uniform structure and a comparatively high cross-linking degree.
2. Description of the Prior Art
Plastics are required to be durable when they are used as materials in industry, agriculture, engineering, construction, or the like. Therefore, intensive research has been carried out to increase the durability of plastics.
In recent years, however, there has been an increasing demand to develop materials which exhibit controlled natural degradation after use in order to prevent environmental pollution due to plastic waste. In general, high molecular weight compounds gradually degrade when exposed to sun light for several years, and, finally, they are degraded to such a degree that they no longer maintain their original form. This is attributable to the fact that sun light comprises irradiation having the necessary wavelength and intensity to split high molecular weight chains. The quantum yield of the molecular bond split is generally as low as 10.sup.-3 to 10.sup.-5. However, one year's exposure to sun light carries enough light energy to split ordinary high molecular weight chains, and hence high polymers are degraded, after accumulated degradation for several years, to such a degree that their original form is destroyed by a slight external force.
Ordinary plastics such as polyethylene, polypropylene, polystyrene, etc., are not easily degraded, when pure, by sun light having wavelength distribution of 290 to 400 m.mu. since they do not absorb radiation of longer than 300 m.mu. in wavelength. Therefore, in order to raise this quantum yield, it is necessary to add a photosensitizer absorbing radiation of longer than 300 m.mu. in wavelength to thereby expand the sensitization wavelength region.
Thus, research has been conducted on adjusting the degree of photodegradability using various photosensitizing agents. In the case of using such agents, it still takes several hours to several months to make the plastics brittle.
In general, absorption of light by molecules is necessary for a photochemical reaction. For the effective absorption of light by molecules of a material to be irradiated, the irradiating light must be well transmitted through the whole material, i.e., a totally uniform and sufficient degree of cross-linking can be attained unless the UV rays are transmitted through the material uniformly and sufficiently. Taking the transmitting capability of UV rays into consideration, it is unavoidable that the cross-linking degree becomes non-uniform in the depthwise direction. The upper limit of the amount of UV irradiation is that which is necessary to cause photodegradation of the material to be irradiated. The efficiencies of transmission and absorption of UV rays and of cross-linking at this irradiation level mainly decide the maximum thickness of practically producable cross-linked materials, and, therefore, foamed products. That is, when the efficiencies of transmission and absorption of UV irradiation or of cross-linking is low, the surface layer becomes over cross-linked, while the inner "core" is under cross-linked, resulting in inferior foamed products. 1,2-polybutadiene transmits UV rays of longer than 240 m.mu. well. The percent transmission thereof is double, or more, that of polyethylene.
In addition, since 1,2-polybutadiene is liable to be photocross-linked based on the vinyl groups in its side chains, it has the merit that comparatively thick uniform cross-linked materials can be obtained. However, in practice, various additives such as fillers, colorants, antioxidants, lubricants, foaming agents, etc., are usually compounded therewith, and these materials generally scatter, obstruct or absorb UV rays to prevent transmission to inner portions and reduce the efficiency of photosensitizers. Therefore, with opaque materials which contain various additives it is difficult to attain a uniform and sufficient cross-linking degree.
In order to obtain cross-linked foamed products of a comparatively high expansion ratio, it is necessary to compound a foaming agent therewith in a correspondingly large amount. The foaming agent generally scatters, obstructs or absorbs UV rays and greatly reduces the efficiency of the transmission or the response of the material being irradiated thereto. Therefore, it is difficult to obtain a uniform cross-linked material having a comparatively high cross-linking degree from foamable materials containing a large amount of a foaming agent so that comparatively thick, uniformly foamed products of a high expansion ratio are obtained only with difficulty. Thus, compounding ingredients which do not prevent the transmission of UV rays and a photosensitizing action, rather which accelerate these effects would be especially valuable.
Cross-linking of rubbery plastics or rubber includes chemical cross-linking using an organic peroxide or a sulfur compound and physical cross-linking using ionizing radiation; the latter conveniently being utilized for the production of cross-linked foamed products.
Physical cross-linking enables to design a process for continuously producing foamed products of good quality having uniform cells with a desired expansion ratio since it does not require heat and pressure to cause cross-linking as does chemical cross-linking.
On the other hand, chemical cross-linking attains cross-linking through the thermal decomposition of a cross-linking agent, and the process involves steps at various temperatures, i.e., softening point of the base resin .ltoreq. kneading temperature &lt; temperature at which thermal decomposition of the cross-linking agent is initiated .ltoreq. temperature at which the thermal decomposition of a foaming agent is initiated .ltoreq. cross-linking and foaming temperature. In order to obtain foamed products having a uniform, fine cellular structure, it is generally necessary to conduct cross-linking prior to foaming, and thermal cross-linking takes a comparatively long period of time. This is the main factor which inhibits the practice of a simple, highly productive cross-linking process as with physical cross-linking.
However, cross-linking using ionizing radiation involves high equipment cost for related equipment to shield radiation harmful to humans as well as for the main equipment. In addition, such a process is not advantageous from the viewpoint of the steps involved.
Japanese Patent Publication No. 10298/71 describes a process for cross-linking and foaming by irradiating polyethylene with ultraviolet rays of low energy. However, pure polyethylene does not absorb radiation having a wavelength of 150 m.mu. or longer, and has a molecular structure which is not influenced by ultraviolet rays of 290 to 400 m.mu. in wavelength. Actually, the poor light resistance thereof is attributable to the absorption region in the longer wavelength region due to processing impurities which contaminate the resin and carbonyl groups formed by oxidation upon thermal hysteresis. Still, polyethylene per se is essentially non-responsive to light in the ultraviolet region. Therefore, even in the process of shifting the absorption wavelength region by the addition of a photosensitizer, ultraviolet rays travel through the materials to be irradiated only a short distance, resulting in non-uniform cross-linking. Also, cross-linking processes require a comparatively long period of time, so that the production of cross-linked foamed products has not extensively been put into practice.
Conventional processes for producing rubber foamed products are as follows. Rubber foamed products can generally be classified into foam rubbers and sponge rubbers. Processes for producing the former are roughly classified into: (1) a sodium silicofluoride process; (2) a thermal coagulating process; and (3) a low temperature coagulating process, according to the process for coagulating the latex, while processes for producing the latter are roughly classified into (A) foaming by mechanical stirring; (B) decomposing a foaming agent; (C) injecting pressurized gas, followed by the removal of pressure; and (D) extracting soluble materials, according to the foaming type, or (a) press curing; (b) autoclave curing; and (c) air vulcanization, according to the type of curing rubber. (A),(B) and (C) are in many cases applied to (c), (a) and (b), respectively, with the application of pressure being a great factor. In any case, a cross-linking process which is required in the production of rubber foamed products involves thermally cross-linking after mechanically compounding a cross-linking agent into the rubber, and requires a comparatively high temperature and a long time. Therefore, various compounding ingredients or modified production steps have been considered, but so far there has been no fundamental use of thermal cross-linking procedures.
1,2-polybutadiene has in every unit a hydrogen atom and a vinyl group both bonded to a tertiary carbon atom at the allyl position, and is therefore, liable to be activated by high energy source such as heat and light. For example, 1,2-polybutadiene comparatively easily undergoes cross-linking and cyclization by short wavelength irradiation (not longer than 350 m.mu.) and undergoes hardening type degradation. Therefore, thin film 1,2-polybutadiene materials easily become brittle when exposed for several weeks to summer sunlight, and become destroyable by wind force. Due to this property, 1,2-polybutadiene belongs to the class of photodegradable plastics.
It is known that, with a 50.mu. thick 1,2-polybutadiene film (trade name: JSR RB-820; made by Japan Synthetic Rubber Co., Ltd.) the specific radiation amount necessary for the hardening degradation is 120 mw.multidot.hr/cm.sup.2 or less when the wavelength is 254 m.mu., and 250 mw.multidot.hr/cm.sup.2 for 312 m.mu. and 600 mw.multidot.hr/cm.sup.2 for 352 m.mu..